واقي الشمس مهم.

A common Levantine-style pronunciation is:

wāʔi sh-shams mhimm

A few notes:

• ق in واقي is often pronounced as a glottal stop ʔ in many urban Levantine accents, so wāʔi.
• In some speakers’ accents, ق stays q, so you may also hear wāqi.
• الشمس is not usually pronounced al-shams. Because ش is a sun letter, the l of ال assimilates, so it sounds like ash-shams / esh-shams / ish-shams, depending on the region.
• مهم is often said as mhimm in everyday speech, though a more careful pronunciation is closer to muhimm.

Because Arabic usually leaves out the present-tense verb to be.

So:

• واقي الشمس مهم = Sunscreen is important

Literally, it looks like:

• sunscreen important

This is completely normal in both Standard Arabic and Levantine. If you want past or future, then a verb appears, for example:

• كان واقي الشمس مهم = Sunscreen was important

واقي الشمس is a noun phrase, and the whole sentence is a nominal sentence.

Breakdown:

• واقي الشمس = the topic / subject-like part
• مهم = the predicate, describing it

Inside واقي الشمس, you have a structure called iḍāfa (construct phrase), which is very common in Arabic.

So it works like:

• واقي = protector / protective thing
• الشمس = the sun

Together, this gives the idea of sun protector, which naturally means sunscreen / sunblock.

Arabic often builds compound nouns this way.

Because in an iḍāfa structure, the first noun normally does not take الـ.

So:

• واقي الشمس = correct
• الواقي الشمس = not correct in this structure

In iḍāfa:

• the first noun stays without الـ
• the second noun can be definite, and if it is, the whole phrase becomes definite

That is why الشمس has الـ, but واقي does not.

Because ش is one of the sun letters.

When الـ comes before a sun letter, the l sound is not pronounced. Instead, the next consonant is doubled.

So:

• written: الشمس
• pronounced: ash-shams / esh-shams / ish-shams

This happens in many words, not just this one. For example:

• الشارع → ash-shāriʿ
• الزلمة → az-zalame

So in your sentence, this is why you hear sh-shams, not al-shams.

Because واقي الشمس is treated as masculine singular.

The important point is that agreement follows the head noun, which here is واقي, not الشمس.

So even though:

• الشمس is grammatically feminine

the full phrase is headed by:

• واقي, which is masculine

That is why the adjective/predicate is:

• مهم = masculine

not:

• مهمة = feminine

This is a very common thing learners trip over: the gender of the second noun does not control the agreement here.

It can mean either, depending on context, but here it most naturally means:

• Sunscreen is important

Arabic and English do not match articles exactly word-for-word. In Arabic, a phrase can be definite in form and still be used in a general, category-level way.

So واقي الشمس مهم can naturally express a general truth, like English:

• Sunscreen is important

But in the right context, it could also mean:

• The sunscreen is important

Context tells you which meaning is intended.

It is understandable and natural enough, but it sounds a bit neutral / standard-like rather than super casual.

In everyday Levantine, people might also say things like:

• سنسكرين مهم
• صن بلوك مهم
• كريم واقي من الشمس مهم

So the given sentence is good and correct, but in daily conversation you may also hear borrowed words from English, especially for modern products.

Yes, you might.

In real-life speech, Arabic speakers are not always perfectly consistent with product-type compounds, so you may hear:

• واقي الشمس
• واقي شمس
• كريم واقي من الشمس
• سنسكرين

If you want to stay close to the sentence you were given, use واقي الشمس. It is clear and widely understandable.

مهم is an adjective meaning important, but in this sentence it functions as the predicate of the nominal sentence.

So it is not directly sitting before a noun like in English important sunscreen. Instead, it comes after the noun phrase and tells you something about it:

• واقي الشمس مهم = Sunscreen is important

This pattern is extremely common in Arabic:

• الجو حلو = The weather is nice
• الأكل جاهز = The food is ready
• الدرس سهل = The lesson is easy